Efficient silicon integrated four-mode edge coupler for few-mode fiber coupling

Here, we designed a broadband, low loss, compact, and fabrication-tolerant silicon-based four-mode edge coupler, composed of a 1 x 3 adiabatic mode-evolution counter-taper splitter and a triple-tip inverse taper. Based on mode conversion and power splitting, the proposed structure can simultaneously realize efficient mode coupling from TE0, TM0, TE1, and TM1 modes of multimode silicon waveguides to linearly polarized (LP), LP01,x, LP01,y, LP11a,x, and LP11a,y, modes in the few-mode fiber. To the best of our knowledge, we proposed the first scheme of four LP modes coupling, which is fully compatible with standard fabrication process. The 3D finite-difference time-domain simulation results show that the on-chip conversion losses of the four modes remain lower than 0.62 dB over the 200 nm wavelength range, and total coupling losses are 4.1 dB, 5.1 dB, 2.1 dB, and 2.9 dB for TE0-to-LP01,x, TM0-to-LP01,y, TE1 -to-LP11a,x, and TM1-to-LP11a,y, respectively. Good fabrication tolerance and relaxed critical dimensions make the four-mode edge coupler compatible with standard fabrication process of commercial silicon photonic foundries.